A need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. A need also exists for a replacement for the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display devices that incorporate a light valve that uses as its light control element a spatial light modulator. Spatial light modulators are typically based on liquid crystal material as described in U.S. Pat. No. 4,813,771, entitled "Electro-Optic Switching devices using Ferroelectric Liquid Crystals," but may also be based on arrays of moveable mirrors as described in U.S. Pat. No. 4,954,789, entitled "Spatial Light Modulator."
Liquid crystal-based spatial light modulators are available in either a transmissive form or in a reflective form. The transmissive spatial light modulator is composed of a layer of a liquid crystal material sandwiched between two transparent electrodes. The liquid crystal material can be either ferroelectric or nematic type. One of the electrodes is segmented into an array of pixel electrodes to define the picture elements (pixels) of the transmissive spatial light modulator. The direction of an electric field applied between each pixel electrode and the other electrode determines whether or not the corresponding pixel of the transmissive spatial light modulator rotates the direction of polarization of light falling on the pixel. The transmissive spatial light modulator is constructed as a half-wave plate and rotates the direction of polarization through 90.degree. so that the polarized light transmitted by the pixels of the spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
Reflective liquid crystal-based spatial light modulators are similar in construction to transmissive liquid crystal-based spatial light modulators, but use reflective pixel electrodes and have the advantage that they do not require a transparent substrate. Accordingly, reflective spatial light modulators can be built on a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from the input video signal. A reflective light valve has the advantage that its pixel electrode drive circuits do not partially include the light modulated by the pixel. This enables a reflective light valve to have a greater light throughput than a similar-sized transmissive light valve and allows larger and more sophisticated drive circuits to be incorporated.
As with the transmissive spatial light modulators, the direction of an electric field (in this case between the transparent electrode and the reflective electrode) determined whether or not the corresponding pixel of the reflective spatial light modulator rotates through 90.degree. the direction of polarization of the light falling on (and reflected by) the pixel. Thus, the polarized light reflected by the pixels of the reflective spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
The resulting optical characteristics of each pixel of both the transmissive and reflective liquid crystal-based spatial light modulators are binary: each pixel either transmits light (its 1 state) or absorbs light (its 0 state), and therefore appears light or dark, depending on the direction of the electric field.
Spatial light modulators based on arrays of moveable mirrors are typically arranged so that the mirror of each pixel has a resting position and a powered position. The resting position is the position the mirror takes when its control mechanism is unpowered. The powered position is the position the mirror takes when power is applied to its control system. When the mirror is in one of the resting position or powered position, it is configured so that light illuminating the mirror is reflected towards an output. In the other position, the mirror is configured so that light illuminating the mirror is reflected away from the output. The resulting optical characteristics of each pixel of moving mirror-based spatial light modulators are binary: each pixel either reflects light toward the output (its 1 state) or away from the output (its 0 state), and therefore appears light or dark, depending on the power condition to the control mechanism.
To produce the grayscale required for conventional display devices with either liquid crystal-based or moving mirror-bases spatial light modulators, several techniques are known in the art. These including time domain grayscale control, light source intensity grayscale control, and a hybrid of time domain and light source intensity domain grayscale control.
With time domain grayscale control, the apparent brightness of each pixel is varied by temporally modulating the 0 state and 1 state of each pixel. The level of gray is controlled by defining a basic time period that will be called the frame period of the spatial light modulator and controlling the duration of the 1 state relative to the duration of the 0 state during the frame period. This determines the apparent brightness, or grayscale, of the pixel.
With time domain control, the frame period for a given pixel is typically divided into time elements associated with a binary weighted value. FIG. 1 depicts time domain control for a pixel given 4-bit grayscale data, corresponding to 16 levels of gray. The figure is a graph with relative light intensity shown on the Y-axis, and time in terms of a time period shown on the X-axis. The light source is depicted as having a constant relative intensity of 15/4ths. The frame period is divided into four time slices A, B, C and D, the relative duration of each period corresponding to the relative value of each of the four digits in a four-digit binary number, such as 1111. Thus, the relative durations correspond to the relative values of the binary numbers 1000, 0100, 0010, and 0001 or their decimal equivalents 8, 4, 2, and 1. Since the sum of these numbers is 15, the four time slices A-D have durations of 8/15, 4/15, 2/15, and 1/15 of a frame period, respectively.
By selectively setting the pixel to either its 1 state or its 0 state during each of the four time slices, any of the 16 levels of gray can be selected as is shown in the following
TABLE 1 Decimal Binary Pixel State Pixel State Pixel State Pixel State Grayscale Grayscale First Second Third Fourth Level Level Period Period Period Period 0 (black) 0000 0 0 0 0 1 0001 0 0 0 1 2 0010 0 0 1 0 3 0011 0 0 1 1 4 0100 0 1 0 0 5 0101 0 1 0 1 6 0110 0 1 1 0 7 0111 0 1 1 1 8 1000 1 0 0 0 9 1001 1 0 0 1 10 1010 1 0 1 0 11 1011 1 0 1 1 12 1100 1 1 0 0 13 1101 1 1 0 1 14 1110 1 1 1 0 15 (white) 1111 1 1 1 1
In practice, the frame period duration may be about 1/60 second (approximately 16,640 .mu.sec) which corresponds to a refresh rate of 60 Hz typically found in computer displays. In addition, grayscale is more typically defined by 8-bits of data than 4-bits of data allowing 256 levels of gray to be defined instead of 16 levels of gray. Using the time domain grayscale control as just described, the frame period would be broken in eight time slices with durations of 8320 .mu.sec, 2080 .mu.sec, 1040 .mu.sec, 520 .mu.sec, 260 .mu.sec, 130 .mu.sec, and 65 .mu.sec, respectively. By selectively setting the pixel to either its 1 state or its 0 state during each of the eight time slices, any of the 256 levels of gray can be selected.
One disadvantage of this type of time domain grayscale control, however, is that the switching speed of the pixel in many of today's light valves is not fast enough to provide a well-defined 0 state or 1 state in less than about 60 .mu.sec. This means that it is difficult to provide more than 256 levels of gray without lengthening the frame period. It also means that 256 levels of gray is difficult to provide when the frame period becomes shorter than about 1/60 of a second as when the refresh rate is increased.
Another disadvantage of this type of domain grayscale control is the minimal time allowed for grayscale data loading during the shortest duration time slices. The magnitude of this problem is not clear from the above description of the operation of a single pixel. In fact, each spatial light modulator usually includes an vast number of individual pixels. For example, a spatial light modulator with a 1,200.times.1,600 pixel array includes 1,920,000 individual pixels. Each of these pixels has individual grayscale data that must be loaded before each change of pixel state. Loading the vast quantity of grayscale data for the next pixel state in the time required for the shortest duration time slices is often very difficult and may require higher cost electronic components and designs, particularly when more than 8 bits of grayscale control are desired.
A second type of prior art grayscale control is light source intensity domain grayscale control. With light source intensity domain grayscale control the frame period for a given pixel is typically divided into equal time elements each associated with a light intensity with a binary weighted value as depicted in FIG. 2. This figure depicts light source intensity domain control for a pixel given 4-bit grayscale data, corresponding to 16 levels of gray with axises that correspond to those found in FIG. 1. The frame period is divided into four time slices E, F, G and H, each with a duration equal to 1/4 frame period. The relative light intensity during each of the four time slices E-H corresponds to the relative value of each of the four digits in a four-digit binary number. Thus, the relative light source intensity correspond to the relative values of the binary numbers 1000, 0100, 0010, and 0001 or their decimal equivalents 8, 4, 2, and 1 as shown.
Light source intensity grayscale control has traditionally been done using fast-acting light sources, and the pixels have been switched to a 0 state during the light source intensity rise-time and decay-time to prevent the transient intensity light generated by the light source during the rise-time and decay-time from reaching the light output and affecting the grayscale level. Rise-time is the time between setting the input of a light source (typically current or voltage control) to the desired higher level and the light source intensity reaching that higher level as a steady state. Similarly, decay-time is the time between setting the input of a light source to the desired lower level and the light source intensity reaching that lower level as a steady state. Certain fast-acting light sources such as LEDs and lasers lend themselves to light source intensity grayscale control because they are relatively easy to modulate between the required intensity levels using current controls know in the art. The also have very short rise-times and decay-times relative to the duration of the frame period.
As previously described, by selectively setting the pixel to either its 1 state or its 0 state during each of the four time slices, any of the 16 levels of gray can be selected as is shown in Table 1. In practice, to achieve 256 levels of gray in a frame period having a duration of 1/60 second (approximately 16,640 .mu.sec.), with light intensity domain grayscale control, the frame period would be divided into eight portions, each with a duration of 2080 .mu.sec.
One advantage of light source intensity grayscale control over time domain grayscale control is that the shortest time slice of the frame period is substantially longer in light intensity grayscale control than in time domain grayscale control. For example, the shortest time slice for the light intensity grayscale control shown in FIG. 1 is 1/4 of a frame period while the shortest time slice for the time domain grayscale control shown in FIG. 2 is 1/15 of a frame period. Thus the shortest time slice in light intensity grayscale control has a duration nearly four times that of the shortest time slice in time domain grayscale control. The ratio of the shortest period for time domain grayscale control to the shortest period for light intensity domain grayscale control becomes even larger as more bits of grayscale are added. This effect is shown in Table 2, below.
TABLE 2 No. of Shortest Period Shortest Period - Ratio of Shortest Gray- No. of - Time Domain Light Intensity Period scale Levels (% Frame Domain (Time Domain: Bits of Gray Period) (% Frame Period) Light Intensity) 4 16 6.67 25.0 1:3.82 8 256 0.392 12.5 1:31.9 16 65,536 0.00153 6.25 1:4,085
The substantial lengthening of the duration of the shortest time slice allows more time for the pixels to actually switch from one state to another and for the grayscale data to be loaded for the next pixel state. Consequently, light source intensity grayscale control allow more grayscale bits can be used, shorter frame periods to be used, and less expensive electronic components and designs can be used than would be possible with time domain grayscale control.
Light source intensity grayscale control, however, has not traditionally been used in some display applications that require slow-acting, high intensity light sources, such as arc-lamps. Rather than quickly achieving the long steady-state intensity levels used by light source intensity grayscale control, these slow-acting, high intensity light sources have rise times and decay times that are a substantial fraction of a frame period or may even exceed the duration of a frame period.
A third type of prior art grayscale control is a hybrid of time domain grayscale control and light source intensity domain grayscale control. Hybrid grayscale control is typically used when the light source intensity is controllable, but where the maximum intensity is reached before the largest of the binary weighted values can be achieved. With hybrid grayscale control the frame period for a given pixel is typically divided into several equal time elements and several time elements that have durations with binary weighted values. Each of the time elements of equal duration are associated with a light intensity with a binary weighted value as with the previously described light source intensity grayscale control.
FIG. 3 is a graph depicting hybrid grayscale control for a pixel given 4-bit grayscale data, corresponding to 16 levels of gray. The axes of the graph correspond to the axises in FIGS. 1-2. For purposes of this figure, it is presumed that the light source has a maximum relative intensity level of 5. The frame period is divided into four time slices J, K, L, and M, with the last three time slices K-M each having durations of 1/5 frame period. In contrast, the first period J has a duration twice that of the other three time slices K-M. During the first two time slices J-K the relative light intensity is at the presumed maximum level of 5 with each following period having a value of half the preceding period. Thus, the intensity of the light source during the third period L is 5/2 and the intensity of the light source during the third period M is 5/4.
Like light source intensity grayscale control, hybrid grayscale control has traditionally been done using fast-acting light sources, and the pixels have been switched to a 0 state during the light source intensity rise-time and decay-time to prevent the transient intensity light generated by the light source during the rise-time and decay-time from reaching the light output and affecting the grayscale level. The same light sources that lend themselves to light source intensity grayscale control are typically used in hybrid grayscale control as well for the same reasons. By selectively setting the pixel to either its 1 state or its 0 state during each of the four time slices, any of the 16 levels of gray can be selected as is shown in Table 1.
Hybrid grayscale control offers the advantage of lengthening the duration of the shortest time slices during the frame period, but to a lesser extent than can be achieved with light source intensity modulation alone. It also offers a grayscale control solution that allows the use light sources with limited ranges of usable intensity modulation.
Like light source intensity grayscale control, hybrid grayscale control has not traditionally been used in some display applications that require slow-acting, high intensity light sources, such as arc-lamps. Rather than quickly achieving the long steady-state intensity levels used by light source intensity grayscale control, these slow-acting, high intensity light sources have rise times and decay times that are a substantial fraction of a frame period or may even exceed the duration of a frame period.
Consequently, what is needed a method of grayscale control that provides the advantages of lengthening of the shortest time slices in a frame period while allowing the use of slow-acting light sources.